Power sources suitable for use in space communication satellite applications require a long life and high energy density secondary battery. Nickel-hydrogen batteries which have high energy densities are being developed to replace nickel-cadmium batteries utilized for satellite applications.
One of the most important life-limiting factors of these batteries is the separator material. The required separator properties include good chemical stability in aqueous KOH electrolyte, good wettability to the electrolyte, good electrolyte retention, proper gas permeability and proper mechanical strength. The separator of a nickel-hydrogen battery requires high-temperature stability in addition to the properties mentioned above. This additional requirement is due to local hydrogen-oxygen recombination which generates high temperature regions in the separator.
The state-of-art nickel-cadmium batteries for certain space applications use nylon or polypropylene felts as the separator material. Nylon has limited chemical stability in the electrolyte. Polypropylene is intrinsically non-wettable in the aqueous electrolyte even though it is chemically very stable. It requires, therefore, a special treatment to be made wettable and usable as a separator. A problem, however, is that the surface of the polypropylene does not stay wettable for a long enough period of time. For a nickel-hydrogen battery, these organic separators are unacceptable because of the requirement of the high temperature stability in addition to the chemical instability.
An asbestos separator has been used in the nickel-hydrogen batteries. However, it also apparently has a chemical stability problem. The only acceptable separators for use in nickel-hydrogen batteries that the present authors are aware of are those described in U.S. Application Ser. No. 948,119 and U.S. Application Ser. No. 914,461.